Google has announced a handful of “search engine of the future” items. One involves the international expansion of the Knowledge Graph and the addition of Knowledge Graph results to search suggestions. One is about improved Voice Search for iOS. More on these things shortly.

Perhaps the most interesting of Google’s announcements is the addition of Gmail inbox content to Google’s search results. Frankly, I’m surprised Google didn’t do this a long time ago, though it will likely ruffle some feathers among the privacy advocates.

Google is introducing the feature on a totally opt-in basis, however. They’re offering a limited trial where you can sign up to get info from your Gmail from the main Google search box.

“Sometimes the best answer to your question isn’t available on the public web—it may be contained somewhere else, such as in your email,” Google’s Amit Singhal said in a blog post. “We think you shouldn’t have to be your own mini-search engine to find the most useful information—it should just work. A search is a search, and we want our results to be truly universal. So we’re developing a way to find this information for you that’s useful and unobtrusive, and we’d love your feedback.”

“We’re working on some even more useful features,” he said. “For example, if you search for [my flights] we will organize flight confirmation emails for any upcoming trips in a beautifully easy-to-read way right on the search results page.”

Earlier this year, Google consolidated its privacy policies into one main policy spanning across Google products, enabling the company to transfer data from one product to the next, under one Google account. This was likely a factor in turning this feature into a reality, and I’d expect more such features to make their way to the public eye, incorporating data from even more Google products into search results.

When Gmail first came out, it was revolutionary because of the amount of storage it gave users. Over time, many users have kept ridiculous amounts of email on file, available from the Gmail search box. Now, all of that content can be surfaced right from a search box. If Google can get the relevancy right, this could be a major step forward for Google Search, provided people actually use it.

It will be interesting to see if Google keeps the feature an opt-in option in the long term.

This feature seems like it would make better use of the “Search Plus Your World” title than the actual Search Plus Your World feature Google launched earlier this year.